The present invention relates to a process for preparing a pyrolytic carbon film, a graphite intercalation compound and a doped carbon film.
The graphite has such unique properties associated with its layer structure such as anisotropies in thermal and electrical conductivity. However, the synthesis of graphite requires extreme conditions of pressure and temperature due to the fact that it has an inaccessible melting point and an extremely low sublimation pressure. For example, so called HOPG is prepared by decomposing a gaseous hydrocarbon (e.g. methane) at 2,000.degree. C. and then hot pressing the resulting pyrolytic material at still a higher temperature. It is now well known that most carboneceous materials are graphitized when they are subjected to a heat treatment at temperatures above 2,500.degree. C. On the other hand, there have been many efforts to prepare pyrolytic carbons, at low decomposition temperatures utilizing dehydrogenation, dehydrohalogenation, decarbonic acid, and the dehydration of selected hydrocarbons. However, the carbon deposits thus obtained are of such a poorly ordered state that they are insufficient to provide anisotropic materials or device made therefrom. There also been known carbon fibers which are obtained by the heat treatment of a fibrous polymer compound at high temperature. These carbon fibers are widely used for structural materials, but their instability in physical properties debases their usefulness for new electronic materials or devices utilizing anisotropic electrical and thermal conductivity. They also lack reproducibility.
To modify graphite leads to the establishment of a variety of degrees of anisotropy, there have been studied many kinds of graphite intercalation compounds (GIC) which are achieved by allowing metal atoms, metal halides or acids to be inserted between adjacent graphite layers of a host graphite material. However, the common method such as the two-zone vapor transport technique and the electrochemical reaction method, as well as other novel methods essentially achieved by direct contact of liquid or gaseous species with host graphite, was applied to such limited reagents that have a low melting point or a high vapor pressure. Most of these compounds with graphite elaborated by these methods are not only unstable but are also sensitive to heat. There has not been reported any air stable GIC in which the intercalant is fixed firmly, nor a practical utilization of the anisotropic properties involved in the new electronic device.
Impurity doping is another method of controlling the degree of anisotropy or the type of conductivity (P-type or N-type). But the fact that graphite is thermodynamically very stable refutes the diffusion technique often used in the fabrication of doped silicon or a germanium semiconductor. Accordingly, doped graphite has not been reported.